In modern drilling practices, the drilling fluid (or mud) acts as the medium for primary well control. Two major well control issues are kicks and drilling fluid (i.e., drilling mud or mud) losses. A kick refers to an event in which an uncontrolled influx of fluids (e.g., oil, gas) from the formation into the wellbore. In extreme cases, the oil and gas escape from the wellbore into open air (i.e, a gusher), causing catastrophic events like fires and explosions. The drilling fluid fills the wellbore, creating a pressure gradient that is larger than the formation pressure gradient (a.k.a., pore pressure gradient) so that the formation fluid is locked in the formation during the drilling process.
On the other hand, if the pressure gradient of the drilling fluid is too high and exceeds the fracture pressure gradient of the formation (i.e., the pressure at which the formation starts to fracture), the drilling fluid may penetrate the formation, causing drilling fluid loss and even collapsing the borehole. In such instances, the formation needs to be protected by casings, which is lowered down through the borehole. A few such casings would quickly reduce the size of the wellbore at the well bottom, rendering it too small for industrial production. Accordingly, the pressure gradient of the drilling fluid shall stay between the formation pressure gradient and the fracture pressure gradient (i.e., the drilling window).
As oil and gas explorations venture into more complex geological conditions, such as in deep sea oil explorations, the drilling window becomes narrower and more irregular. Kicks not only come from drilling through layers of formations having different formation pressure gradients, but also are frequently induced by routine operations such as tripping. Therefore, faster and more accurate control of the drilling fluid pressure gradient becomes more important.
Managed pressure drilling (MPD) is an enhanced drilling method that addresses some of the challenges described above. Instead of using a drilling fluid system that is open to the air, the MPD closes the drilling fluid loop to the air using equipment including a rotating control device (RCD), drilling string non-return valves (NRV), and a dedicated choke manifold. Simply put, the additional equipment seals off the drilling fluid from the air and exerts an actively controlled back pressure to the drilling fluid. The back pressure allows the operator to use a lighter drilling fluid so that drilling may occur at a pressure gradient closer to the formation pressure gradient, effectively extending the operable drilling window. In addition, the back pressure can be quickly adjusted upon the detection of any sign of kicks or fluid losses, more effectively controlling the well conditions, such as the Bottom Hole Pressure (BHP). BHP is the pressure at the bottom of a well. MPD enables a stable BHP and avoids oscillations of the BHP during the drilling.
Furthermore, better pressure control also reduces incidences of formation fracture and consequently reduces or avoids complex casing operations. As a result, the well bottom maintains a size large enough for production purposes. Accordingly, an increasing number of drilling operations are adopting the MPD method, especially in offshore deepwater drilling operations.
Despites the benefits of using MPD drilling systems, major concerns such as kicks and mud loss still exist in tight drilling windows. Sensitive kick detection methods, comprehensive well control procedures and adequate kick processing equipment (separators, flare booms, etc), are critical elements of prudent MPD well design. Therefore, there is a need for methods and equipment for optimizing drilling and well construction for the MPD drilling system.